1) Field of the Invention
The present invention relates to an optical device and a light control method suitably used in a field of optical communication, particularly to an optical device and a light control method suitably used in a wavelength division multiplexing communication system.
2) Description of the Related Art
In a network of wavelength division multiplexing (WDM) communication, a plurality of nodes are disposed in a mesh-like state, and in each of the nodes, and it is arranged so that wavelength-division multiplexed light beam is separated based on the wavelength, and set up which node other than its own should be connected based on the wavelength; thereby, light paths based on the wavelength for connecting two specific nodes can be set up freely. Owing to this, the net configuration of a mesh-like network can be changed freely.
By employing the light path based on the wavelength, which is set up as described above, the following problems are eliminated; i.e., time delay necessary for converting the light beam into electricity once to read the signals and converting the signals into the light beam again; decrease of through-put due to the limitation of speed in electrical processing; necessity to select a decoder due to difference in decoding method; and further necessity of providing an expensive decoder for decoding the signals encoded with a deep code and soon. Hereinafter, the device, which is used for switching the light path used in a node as described above, will be referred to as “wavelength selection switch”.
In the node, which is disposed in a network of the WDM communication, in addition to the function as described above for switching the light path based on the wavelength, the following functions are also further required; i.e., to separate wavelength-division multiplexed light beam based on the wavelength; to measure the optical power thereof; to control the optical power to a specific value; and to detect faults such as fiber break or the like.
As for the configuration of a wavelength-dividing filter for separating wavelength-division multiplexed light beam based on the wavelength as described above, there are, for example, such configuration in which diffraction grating type wavelength dividing filter and periphery optical system are combined set forth in the patent documents 1 to 4 listed below, or such configuration, in which a diffraction grating type wavelength dividing filter and a periphery optical system are combined set forth in the patent documents 5 and 6.
Patent documents 1 to 4 respectively disclose a wavelength-dividing filter in which a reflective diffraction grating and a periphery optical system are combined. Patent documents 5 and 6 respectively disclose a wavelength-dividing filter in which a transmissive diffraction grating named as VPG (Volume Phase Grating) as a diffraction grating and a periphery optical system are combined. Difference between the filters disclosed in these patent documents 1 to 4 and the filters disclosed in the patent documents 5 and 6 is only a point that the diffraction grating is a reflective type or a transmissive type; thus there is no substantially significant difference.
FIG. 40 schematically illustrates the configuration of a wavelength-dividing filter set forth in the above-mentioned patent documents 1 to 6 being simplified along with basic elements of the periphery optical system. The wavelength-dividing filter shown in FIG. 40 includes a wavelength dividing filter (dividing section) 500-1, a light control section 7 and a multiplexing section (multiplxer) 500-2.
The dividing section 500-1 being the wavelength-dividing filter 500-1 is formed of a diffraction grating and the like as a primary member, and separates an input light beam (input light) 4a to spatially continuous spectrums (light to be used) 4b. That is, the dividing section 500-1 divides the input light beam 4a into the spectrums 4b in which wavelength component is sequentially distributed from the shorter wavelength to the longer wavelength corresponding to the outputted position.
The light control section 7 includes a plurality of optical elements 7-1 to 7-n (in FIG. 40, 5 optical elements 7-1 to 7-5) disposed in parallel that performs control processing on each of the partial wavelength spectrum component with respect to the light beam divided by the dividing section 500-1. The optical elements 7-1 to 7-n may be formed of, for example, a mirror having limited widths or a light-attenuating element having a specific opening.
For example, as shown as an example in FIG. 40, as the optical elements 7-1 to 7-n, light attenuating elements having specific openings are employed, and light beams only that have been filtered out through the light control section 7 are multiplexed again by the multiplexing section 500-2, and being connected to an unshown optical fiber, a WDM light beam (wavelength-division multiplexed light beam) in which the optical power is adjusted is obtained. That is, the light beam within a specific spectrum space in the light beam 4a having a spectrum of which wavelength component has been spatially separated by the dividing section 500-1 can be adjusted in the intensity of transmitted light beam by the optical elements 7-1 to 7-n as the light attenuating elements.
Or, mirrors may be employed as the optical elements 7-1 to 7-n, thereby the light beam is caused to be reflected in a plurality of directions, multiplexed by a plurality of multiplexers 500-2, and connected to a plurality of unshown optical fibers. Thus, the light beam outputted from one optical fiber can be arbitrary inputted to a plurality of other optical fibers based on the wavelength. Accordingly, the mirrors can be caused to function as optical switches (wavelength selection switches) that allow the light beam separated based on the wavelength to propagate in specific directions.
In FIG. 40, a transmissive type optical model is shown as an example. When the dividing section 500-1 and multiplexing section 500-2 are optically connected to each other by the light control section 7 as the mirror, an optical model substantially the same as that shown in FIG. 40 can be configured.
Furthermore, when the above-described optical elements 7-1 to 7-n are configured using photoelectric transfer elements that convert the received light into electrical signals, the light beam, which has been wavelength-divided by the dividing section 500-1, can be monitored to detect faults such as fiber break.
(Patent document 1): Japanese Patent Laid-Open(Kokai) No. 2000-347065
(Patent document 2): Published application Japanese translation of PCT No. 2003-515187
(Patent document 3): U.S. Pat. No. 6,204,946
(Patent document 4): U.S. Pat. No. 6,549,699
(Patent document 5): U.S. Pat. No. 6,108,471
(Patent document 6): U.S. Pat. No. 6,671,428
However, in the above-described example shown in FIG. 40, there reside gaps 7G between the optical elements 7-1 to 7-n constituting mirrors or the like as described above, which are hardly eliminated. Even the light beams divided by dividing section 500-1, the light beams 4c located at the portions of these gaps 7G cannot enter the light control section 7 resulting in unusable light beam (invalid light) 4c. Therefore, there resides such a problem that, compared to the light beam 4a which has been divided by the dividing section 500-1, the waveband of the spectrum of the light beams (light to be used) 4b, which is received by the light control section 7 and utilized, apart thereof is lost. That is, there resides such problem that the spectrum of the wavelength channel is substantially reduced.
Since a part of the spectrum of the light beam, which is received by the above-described light control section 7 and utilized, is lost, such a problem resides in; i.e., a defect is caused in the flatness of the transmission loss characteristics with respect to the wavelength of the waveband in the light beam outputted from the light control section 7.